Lithographic plates and method for manufacture



' Nov. 17, 1964 E. B. RELPH ETAL 3,157,117

LITHOGRAPHIC PLATES AND METHOD FOR MANUFACTURE- Filed 061.- 31, 1957 WET STRENGTH PAPER f Qt!- INVENTORS: EARL a. RELPH HENRY A. DAHL United States Patent 33,157,117 rirnoonnrrnc stares AND METHQD non MANUFAQTURE This invention relates to lithographic plates and to the method for the manufacture of same, and it relates more particularly to a new and improved paper lithographic plate for use in office copy work.

To the present, practically all of the lithographic plates, with the exception of metal plates, have been formed of paper base sheet, preferably in the form of a high wet strength paper, having an insolubilized colloid coating on the surface thereof which is characterized by water receptivity and ink repellency in the surface portions to provide the desired lithographic properties, and which is capable of receiving an imaging material thereon which is ink receptive and water repellent so that the hydrophilic, non-imaged portions of the plate will be wet with Water to repel ink, while the hydrophobic, imaged portions will receive ink in preference to water for transfer to copy sheets through the medium of a transfer member to produce copies. Coated paper lithographic plates which make use of colloids in the preparation of the lithographic surface may be represented by the Shepherd Patent No. 2,154,219 (casein), Worthen Patent No. 2,534,650 (casein), Van Dusen Patent No. 2,542,784 (carboxymethyl cellulose), Ensink Patent No. 2,806,424 (alginates), Beatty Patent No. 2,760,431 (polyacrylate), and the Toland and Bassist Patent No. 2,230,981 (polyvinyl alcohol).

Characteristic of the colloids that are employed in forming the hydrophilic lithographic coating on the paper base sheets are their solubility in aqueous medium to enable formulation into aqueous coating compositions applied to the prepared paper surface, the use of aqueous medium as a carrier for the colloids in the composition applied to coat the paper base sheet, the necessity to insolubilize the colloid applied as a coating onto the base sheet to provide the desired resistance to removal by the Water applied as a repellent onto the sheet, and the need to balance the ink-repellent, water-receptive, hydrophilic characteristics of the surface with the ability to anchor an ink-receptive, water-repellent, imaging material onto the surface of the plate with sufiicient permanence to resist loss of the image before a particular number of copies of good quality have been secured.

Many of these characteristics are difficult to meet, while others of the properties of the colloid and its coating impose problems in the manufacture and use of the plate. For example, the necessity to make use of an aqueous system containing the colloid in solution for coating the paper base sheet imposes many problems in manufacture because the aqueous medium enters the fibers of the sheet and enables impregnation by the coating to the extent that imbalances are set up in the sheet which cause curling and wrinkling and the like difficulties in manufacture and use. To minimize curling, it is sometimes necessary to apply additional coatings to the sheet or to backcoat the sheet, or to make use of such papers or to make use of various combinations of such cures, sometimes without beneficial results.

Because of the water-soluble character of the colloid as applied from the coating composition, it becomes difficult sufiiciently to insolubilize the colloid after application onto the base sheet. As a result, the mere presence of Water on the surface of the sheet in lithographic print 3,l57,ll7 Fatented Nov. 17, 1964 ing causes deterioration of the coating to the extent that.

either copy of poor quality is produced or else an insuificient number of copies are secured before the plate is destroyed. Where, in many instances, ammonium hydroxide is used in the aqueous system to solubilize the colloid in formulation of the coating composition, extensive precautions and sometimes extended aging and drying are required sufficiently to eliminate the ammonium hydroxide to insolubilize the colloid in the coating.

Further, tlre water-loving character of the colloid is in conflict with the ability to cause an ink image to adhere to the surface of the hydrophilic coating in the presence of water. Thus a very significant problem is inherent in the use of colloids as the hydrophilic material in the lithographic coating since the very nature of the hydrophilic surface is to prefer water over ink so that the ink image would ordinarily tend to be displaced by the water with which the plate is constantly wet in use, thereby to result in loss of the image.

It is an object of this invention to produce a pianographic printing plate which is not subject to the limitations heretofore experienced in the use of hydrophilic colloid coatings applied from aqueous systems in the manufacture of the printing plate.

More specifically, it is an object of this invention to produce and to provide a method and materials for producing a paper coated lithographic plate in which the coating becomes insolubilized upon drying; which does not raise curl problems in the coated paper base sheet; which is relatively insensitive to the presence of moisture and high humidity; which can be easily and quickly Wet out in the non-imaged portions of the plate and which is capable of retaining an ink-receptive image. on the surface; which can be applied from a solvent system thereby to avoid imbalance in the paper base sheet; which is capable of being manufactured with a minimum number of process steps and with readily available and inexpensive materials to produce a short or long run plate for the reproduction of copy of good quality.

These and other objects and advantages of this invention will hereinafter appear and, for purposes of illustration but not of limitation, embodiments of the invention are shown in the accompanying drawing, in which- FIGURE 1 is a perspective view of a lithographic plate embodying the features of this invention;

FIGURE 2 is a sectiona elevational view through a portion of the plate shown in FIGURE 1, and taken along the line 2-2 of FlGURE 1;

FIGURE 3 is a perspective view of a lithographic plate embodying a modification in a lithographic plate; and

' FIGURE 4 is a sectional view taken substantially along the line 4-4 of FIGURE 3.

In accordnce with the practice of this invention, a direct image, offset master is fabricated of a paper base sheet iii coated with an alkyd resin embodied in a solvent system, thereby to avoid the necessity for insolubilization of the lithographic surface while at the same time avoiding the difficulties inherent in the use of water as a carrier and the colloid as the adhesive in the manufacture of the lithographic plate. It has been found that use can be made of an alkyd resin as the surface coating in the manufacture of a lithographic plate, notwithstanding the hydrophobic characteristics of the alkyd resins, when use is made of an acid etch, such as oxalic acid, to wet out the surface of the plate prior to its use on the printing press, thereby to convert the hydrophobic characteristics of the surface in the non-imaged portions to a hydrophilic ink-repellent, water-receptive surface.

As the base sheet, use can be made of a treated paper, as represented by the 45-pound Gleamcoat CIS paper marketed by Crocker Burbank, as illustrated by the numeral in FIGURE 1. It is preferred, however, to make use of a high wet strength paper 2.0, such as a paper which has been impregnated or otherwise treated with a water-insolubilizing resin of the type urea formaldehyde, melamine formaldehyde, phenol formaldehyde and the like to increase the wet strength of the paper. When use is made of a treated paper of high wet strength, it is often desirable to make use of a barrier coat 22, such as an ester gum coat or an ester gum-casein coat, to minimize the penetration of water into the paper base sheet and to prevent deterioration or distortion thereof in use. The barrier coat 22 can function also to tie in the lithographic coating 24- of the alkyd resin to the base sheet to produce a composite structure of greater strength and longer life. In addition to the base sheet described, use can be made of paper base sheets having a plurality of resinous or colloid coatings, such as described in the Worthen Patent No. 2,534,650, or as described in the Mullen Patent No. 2,693,145.

As the alkyd resin, use can be made of the condensation reaction products of a polyhydric alcohol as represented by glycol, glycerine, or pentaerythritol and the like with a polybasic acid such as phthalic acid, phthalic anhydride, or maleic acid in substantially balanced proportions of carboxyl to hydroxyl groups. Use can be made of an oil modified alkyd resin of the short or long oil type but, in the preferred modification, it is desirable to make use of an alkyd resin of the type described of the short oil type and in which the oil modifier is selected of castor oil, soya bean oil, linseed oil and the like. A suitable short oil modified alkyd resin is marketed by the General Electric Company of Schenectady, New York, under the trade name Glyptal 2480, or by Archer Daniels & Midland under the trade name Aroplaz 7424 (drying oil), 2570 (non-drying oil), 2520 (short castor oil) or 7310 (short soya oil). Suitable long oil types are marketed by Archer Daniels & Midland under Aroplaz 2503.

It is desirable to make use of a zinc oxide component in the coated composition, preferably in the ratio of about 1-8 parts by weight of zinc oxide to 1 part by weight of the resinous material. In addition to the zinc oxide use can be made of fillers such as calcium carbonate, satin white, clay, diatornaccous earth and the like finely divided inert or siliceous material.

The following will illustrate the practice of the invention:

Example I Coating composition: Parts by weight Castor oil modified alkyl resin (Glyptal 24% lead naphthenate 3.7 6% cobalt naphthenate 3.5 6% calcium naphthenate 4.9 Xylene 275 Zinc oxide 400 Example II Coating composition:

Alkyd resin 100 24% lead naphthenate 3.5 6% cobalt naphthenate 4.0 Benzene 300 Zinc oxide 350 Satin white 50 The metal naphthenates in the foregoing representative compositions are added to accelerate the drying of the oil type alkyd resin. The dryers can be employed in different combinations and in diferent amountsQdepending upon the rate of drying desired. Other dryers conventionally employed for the setting of oils can be used.

The composition is applied onto the wet strength paper 10, as in FZGURE 1, or onto a wet strength paper 20 having a barrier coat 22 on the surface thereof, as in FIGURE 3. The composition is applied in an amount to provide a coating weight within the range of 10-25 pounds per 3000 square feet of surface area, and preferably in amount to provide 18-20 pounds per 3000 square feet of surface area. The applied coating may be allowed to air dry, but it is preferred to accelerate drying by exposure of the coated paper base sheet to an elevated temperature by advancing the coated paper ase sheet through a drying oven at about ISO-200 F. or through a bank of infra-red ray lamps.

No curl will be experienced upon drying because the moisture balance in the paper base sheet remains substantially uniform throughout and the solvent component is substantially completely eliminated almost immediately upon drying.

The coating 12 in FIGURE 1 or 24 in FIGURE 3 that is formed on the surface of the base sheet is insoluble in water without further treatment, thereby to resist deterioration or separation upon exposure to the aqueous wet-out or repellent solutions constantly applied to the surface of the plate during the printing operation.

The plate can be imaged with conventional imaging materials, such as conventional offset ribbons containing a greasy substance for imaging the plate, or by a lithographic imaging crayon of copal or the like, or by a reproducing ink containing an ink-receptive, water-repellent imaging material. In other words, the plate can be imaged by conventional imaging materials of the type heretofore employed for offset plates formed of paper base sheets having a colloid coating as a lithographic surface, as represented by the types heretofore produced.

Before mounting the plate on the press for the reproduction of a multiplicity of copies, the surface of the imaged plate should be treated with a relatively strong acidic substance such as a 12% solution of oxalic acid. Other acids, such as formic acid, hydrochloric acid, or nitric acid can be employed, but oxalic acid in concentrations of from 5-15 appear to give the best results. The acidic material reacts with the zinc oxide or the combination of Zinc oxide and alkyd resin in non-imaged portions of the surface to convert the hydrophobic characteristics of the surface to one that becomes hydrophilic, ink repellent and water receptive. As a result, a conventional fountain solution and ink composition can be used on the press to produce hundreds of copies of good quality. The plate can be formed with clamping holes 16 at one end or with scallops 26 to facilitate clamping on the plate cylinder.

It will be apparent from the foregoing that We have succeeded in producing a direct image plate which can be easily and efficiently manufactured with a minimum number of coatings by comparison with the plurality of coatings built onto the paper base sheet when fabricated of a hydrophilic colloid, as represented in the aforementioned Worthen and Mullen patents. By way of still further improvement, a paper offset master prepared in accordance with the practice of this invention has greater dimensional stability because of the insolubility of the coating, because of the use of a solvent system as compared to an aqueous system for coating, and because of the ability to make use of a base sheet which has high wet strength and which may even be water repellent. The formed plate will be found to have improved stability from the standpoint of the absence of curl or the necessity to apply additional coatings to compensate for curl which characterizes paper plates fabricated with a hydrophilic colloid coating that is applied from an aqueous system. It has better utility from the standpoint of the number of copies of good quality capable of being produced because of the ability to anchor the image onto the hydrophobic and more receptive surface prior to conversion of the non-imaged portions by an acid etch to render the remainder ink repellent and water receptive after the image has been anchored.

It will be understood that changes maybe made in the details of formulation, application and treatment and in the character of the base sheets employed for support of the alkyd coating Without departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. A planographic printing plate, comprising a base sheet and a coating on the surface of the base sheet formed of a water-insoluble, hydrophobic, alkyd resin and a finely divided Zinc oxide present in the ratio of 1-8 parts by weight of zinc oxide to 1 part by weight of the alkyd resin, and means on the plate clamping onto a plate cylinder.

2. The method of'producing a coated paper planographic printing plate, comprising coating the paper With a solvent solution ofa zinc oxide-alkyd resin, drying the plate to set the coating on the surface of the paper, imaging the coated surface to provide an ink-receptive, waterrepellent imaged portion, treating the surface of the coating with an acid which is adapted to convert the hydrophobic characteristics of the coating surface in the nonimaged areas to a hydrophilic, ink-repellent and Waterreceptive surface.

3. The method as claimedin claim 2 inWhich the paper comprises a high Wet strength paper base sheet.

4. The method of producing a coated planographic printing plate, comprising coating a base sheet with a composition having a solvent as the diluent and containing an alkyd resin as the adhesive and a finely divided zinc oxide and inert filler, the zinc oxide being present in the ratio of 1-8 parts by Weight of zinc oxide to 1 part by Weight of the alkyd resin, drying the coating on the surface of the base sheet, imaging the coated surface to provide an ink-receptive, water-repellent, imaged portion, treating the surface of the plate with an acid which is adapted to convert the hydrophobic characteristics of the coated surface in the n011-imaged areas to a hydrophilic, ink-repellent, water-receptive surface.

5. The method of producing a coated planographic printing plate, comprising coating a base sheet with a composition having a solvent as the diluent and containing an alkyd resin as the adhesive, a finely divided zinc oxide and an inert filler, the zinc oxide being present in the ratio of 1 to 8 parts by weight of zinc oxide to one part by Weight of the alkyd resin, drying the coating on the surface of the base sheet,'imaging the coated surface to provide an ink-receptive, water-repellent imaged portion, and treating the surface of the plate with a 5 to 15% oxalic acid solution to convert the hydrophobic characteristics of the coated surface in the non-imaged areas to a hydrophilic ink-repellent, water-receptive surface.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Sugarman: Electrofax, A New Tool for the Graphic Arts, published in The American Pressman, November 1955, pages 33-3 8, page 35 made of record. 

1. A PLANOGRAPHIC PRINTING PLATE, COMPRISING A BASE SHEET AND A COATING ON THE SURFACE OF THE BASE SHEET FORMED OF A WATER-INSOLUBLE, HYDROPHOBIC, ALKYD RESIN AND A FINELY DIVIDED ZINC OXIDE PRESENT IN THE RATIO OF 1-8 PARTS BY WEIGHT OF ZINC OXIDE TO 1 PART BY WEIGHT OF THE ALKYD RESIN, AND MEANS ON THE PLATE CLAMPING ONTO A PLATE CYLINDER. 